Ballast transformers for various types of lighting sources are well known. The ballast receives voltage from a supply and provides the operating voltage and current for the light source. A type of light source operated by a ballast is a high intensity discharge (HID) type lamp, such as mercury vapor, metal halide and sodium vapor lamps. In some applications, it is desired that the screw shell of the HID lamp socket, which is electrically connected to the low potential side of the secondary circuit, be electrically grounded.
A common regulating ballast used at the present time for powering HID lamps is of the constant wattage autotransformer (CWA) type, a schematic diagram of which is shown FIG. 1. Here, the ballast transformer 10 includes a primary winding 12 in an autotransformer configuration. The lower portion of the primary winding, as shown, provides a voltage in additive relationship to the secondary winding 14 to produce the final operating voltage supplied through a capacitor 16 to the HID lamp 18. The magnetic portion of the ballast is schematically shown by the laminations 20 which includes a magnetic shunt 22.
The CWA autotransformer type ballast of FIG. 1 does not provide electrical isolation between the primary and secondary windings. Therefore, it is not possible to electrically ground the lamp screw shells of an HID lamp operated by the CWA style ballast shown in FIG. 1 which operates from polyphase voltage supply systems. The CWA ballast of FIG. 1 is used in many applications requiring regulated ballasts without a requirement for grounding of the screw shells.
In applications that require the HID lamp screw shells to be grounded, constant wattage isolated (CWI) and magnetic regulating ballasts are used. Designs of these types of ballasts are shown in FIGS. 2 and 3. As seen in FIG. 2, the primary winding 12 and the single secondary winding 14 are electrically isolated from each other. The ballast of FIG. 3 is of the magnetic regulating type which includes two separate secondary windings 14-1 and 14-2 which are isolated from the primary winding 12. Magnetic shunts 22 and 22-1 associated with the laminations 20 assist in providing regulating features.
Since the ballast designs of FIGS. 2 and 3 have primary and secondary windings which are electrically isolated, they can be used on polyphase power system where an HID lamp mounting shell is to be electrically grounded. However, neither of these designs has the advantage of an autotransformer in which part of the voltage on the primary side is added to the secondary winding side voltage. Without deriving part of the required secondary voltage directly from the primary winding, additional secondary turns are required for the secondary windings in both of these designs to obtain the same output voltage as in an autotransformer design. Accordingly, the ballasts of FIGS. 2 and 3 are bigger, consume more energy, and are substantially more costly than an autotransformer type ballast.